1. Field of the Invention
This invention pertains generally to a variable gain amplifier, more specifically to a wideband amplifier capable of selecting a plurality of predetermined gains.
In electronic test and measurement instruments such as, for example, an oscilloscope, there is a need for wideband amplifiers to provide a plurality of predetermined voltage amplification factors (or gains) with accurate ratios for switching the sensitivity or other purposes. Such wideband amplifiers are required to provide switchable gains of desired ratios while maintaining other electrical characteristics such as frequency response unchanged.
2. Description of Prior Art
One example of conventional amplifier circuits for such purpose is shown in FIG. 1. The base electrodes of a pair of transistors 12 and 13 define input terminals 10 and 11 to which input signals are applied. Coupled between the emitter electrodes are first coupling resistors 16-17 by way of diodes 14 and 15 and also second coupling resistors 20-21 by way of diodes 18 and 19. As described hereinafter, coupling resistors 16-17 and 20-21 are chosen to have different resistance with each other depending on the desired gains. Junctions of the coupling resistors 16-17 and 20-21 are connected to current source 22 each through current switch transistors 24 and 23, respectively. Current switch transistors 23-24 receive at the base electrodes thereof a control signal through control terminal 25 to alternatively turn either one of the transistors depending upon the relatively high voltage level applied to the base control terminals 25. The collector electrodes of differential transistor pair 12-13 are connected to a positive voltage source each through collector load resistors 26 and 27 and also define output terminals 28 and 29. Although not shown in the drawing, a capacitor or a series combination of a resistor and a capacitor is connected in parallel with at least one of coupling resistors 16-17 and 20-21 to compensate for frequency bandwidth of the amplifier.
In operation, the base voltage of one of current switch transistors 23-24, say transistor 24 is raised to relatively high level to turn transistor 24 on, thereby connecting first coupling resistors 16-17 between the emitters of differential transistor pair 12-13 and disconnecting second coupling resistors 20-21 from the circuit by diodes 18-19. Then, this amplifier constitutes a balanced amplifier having a gain determined by the ratio of collector resistor 26 (or 27) and first emitter coupling resistor 16 (or 17). Increasing now the base voltage of the current switch transistor 23 to a relatively high voltage, transistor 23 turns on to connect second emitter coupling resistors 20-21 between the emitters of differential transistor pair 12-13 while disconnecting first emitter coupling resistors 16-17 from the circuit by diodes 14-15. The gain of the amplifier is, then, determined by the ratio of collector load resistor 26 (or 27) and second emitter coupling resistor 20 (or 21). Consequently, the gain of the differential amplifier can be switched by the factor predetermined by the ratio of first and second emitter coupling resistors by turning on and off the current switch transistors 23 and 24.
The prior art curcuit as shown in FIG. 1 includes the following disadvantages. Firstly, the switching of the emitter coupling resistor tends to cause a change in operation signal level of differential transistor pair 12-13. The amplifier gain can hardly be changed while maintaining the frequency bandwidth unchanged. Gain switching to three or more different values will unavoidably increase the emitter circuit wiring, thereby degrading the high frequency signal response. The diodes interposed in series with the base-emitterjunctions accompany gain changes, drift problem, etc. at different temperatures.